This paper proposes a theory and method for quantitative measurement of the acoustic lens-line focusing ultrasonic (ALLFU) field in its focal spot size and acoustic pressure using the Schlieren imaging technique. Using Fourier transformation, the relationship between the brightness of the Schlieren image and the acoustic pressure was introduced. The ALLFU field was simulated using finite element method and compared with the Schlieren acoustic field image. The measurement of the focal spot size was performed using the Schlieren method. The acoustic pressure in the focal zone of the ALLFU field and the transducer-transmitting voltage response were quantitatively determined by measuring the diffraction light fringe intensity. The results show that the brightness of the Schlieren image is a linear function of the acoustic intensity when the acousto-optic interaction length remains constant and the acoustic field is weak.